1. Field of the Invention
The present invention relates generally to detecting anomalies in microwave penetrable material and, more particularly, to the use of multiple reflections obtained at each of multiple positions in the microwave penetrable material for determining such anomalies. Anomalies of this type include but are not limited to plastic land mines, underground plastic pipes, liquid/foam interfaces, interfaces between geological materials, voids, and the like, which may be found in microwave penetrable environments that may camouflage such anomalies to a high degree.
2. Description of Prior Art
On average, every twenty minutes someone in the world loses a limb to a landmine. Many landmines are made of plastic.
Ground probing radar (GPR) has been used with some success in detecting underground metallic objects, but systems for detecting plastic or nonferrous objects are unreliable and have very little real world detection success for small objects. The environment of the ground provides an effective mask that makes it difficult to distinguish plastic and nonferrous materials electromagnetically. For instance, false detections caused by rocks, tree roots, air pockets, soil inhomogeneity, and miscellaneous buried objects are often a problem. Another problem is the apparent disappearance of a mine at some frequencies caused by either a permittivity match with the surrounding soil or an unfavorable complex addition of reflected energy at the receiver. Additional problems arise from high moisture content in the soil or a layer of water above the mine or plastic object.
Electromagnetic induction techniques generally work well for metallic objects. However, the amount of metal in plastic mines is often very small (only the fuse element, or none at all). This makes detection of plastic mines difficult or impossible. Plastic or PVC pipe cannot be detected in this way.
Infrared detection is another technique commonly used for mine detection. Because there is generally a difference in the heating/cooling rate of the mine as compared to the soil, at certain times of the day a surface spot above the mine can be detected because of a slight temperature difference. However, this technique fails when the day/night temperature changes are minimal and when the mine is buried more than a few inches deep.
Other techniques such as superconducting magnetic field gradiometers, nuclear magnetic resonance imaging, and thermal neutron activation have been used with some success, but all have been shown to be deficient in one respect or another for detecting small plastic land mines. In addition, the equipment using these techniques is heavy, costly, and not amenable for field use in many environments.
Patents related to this area show many attempts to solve the above problems.
U.S. Pat. No. 5,867,117, issued Feb. 2, 1999, to Gogineni et al., discloses an apparatus and method for detecting an object and determining the range of the object. A transmitter, coupled to an antenna, transmits a frequency-modulated probe signal at each of a number of center frequency intervals or steps. A receiver, coupled to the antenna when operating in a monostatic mode or, alternatively, to a separate antenna when operating in a bistatic mode, receives a return signal from a target object resulting from the probe signal. Magnitude and phase information corresponding to the object are measured and stored in a memory at each of the center frequency steps. The range to the object is determined using the magnitude and phase information stored in the memory. The present invention provides for high-resolution probing and object detection in short-range applications. The present invention has a wide range of applications including high-resolution probing of geophysical surfaces and ground-penetration applications. The invention may also be used to measure the relative permittivity of materials.
U.S. Pat. No. 5,592,170, issued Jan. 7, 1997, to Price et al., discloses a frequency-agile, narrow-instantaneous bandwidth radar system that detects objects, and discriminates between different types of objects, from a safe stand-off distance. Transmit circuitry transmits a train of continuous wave signals in a multitude of stepped operating frequencies that illuminates the target area. Return signals from the target area are received through at least a pair of spaced-apart receive antennas. Signal receive/processing circuitry coupled to the spaced-apart receive antennas selectively combines and processes the return signals to identify variations in the received signals indicative of the presence of a specific type of object. At each of the stepped frequencies, the system noise and the clutter of the signals is reduced by averaging and smoothing the incoming data, and the cross-power spectrum at each frequency is calculated. Using the information of the power spectra of all frequencies, the Mahalanobis distance is defined and the presence and classification of a target is determined. Using the information of the cross-power spectra of all frequencies, the location of the mine is determined by the azimuth angle and echo time.
U.S. Pat. No. 4,240,027, issued Dec. 16, 1980, to Larsen et al., discloses a method for electromagnetic analysis of cellular or cell ghost physiology and pharmacology without disrupting the physical integrity of the cell membrane is described. The method utilizes the technique of multi-frequency automatic network analysis and signal processing to derive complex permittivities from the error corrected complex reflection coefficient of cell containing samples at each measured frequency. Complex permittivity at each frequency is then related to the dispersion in dielectric conductivity (a term which includes ohmic and non-ohmic losses) thereby measuring the ion permeability barrier and transport functions of the cell membrane and ion distribution inside of and outside of the cell membrane. The method measures the complex reflection coefficient of a capacitive termination containing a cellular sample as high frequencies are applied. Meaningful data can be developed in the range of frequencies of from 100 KHz to 100 MHz depending upon the exact nature of the cells and the automatic network analyzer used.
U.S. Pat. No. 5,557,277, issued Sep. 17, 1996, to Tricoles et al., discloses a method for imaging substances leaking from underground structures using continuous-wave signals that includes the steps of translating an antenna array over the ground, transmitting a continuous-wave signal into the ground at an array of points, detecting the amplitude and phase of the reflected signal at each point, transforming the reflectance values into the frequency domain, propagating this reflectance spectrum to a predetermined depth, and transforming the propagated spectrum into an image in the spatial domain at that depth. An image representing the underground structure containing the substance may be overlayed on the calculated image to detect differences that represent leakage. Successive images of the same area may be produced over a period of time and the differences compared to determine the rate of leakage.
U.S. Pat. No. 5,819,859, issued Oct. 13, 1998, to Stump et al., discloses an apparatus and method for locating an underground object or structure by employment of a radar-like probe and detection technique. The underground structure is provided with a device which generates a specific signature signal in response to a probe signal transmitted from above the ground. Cooperative action between the probe signal transmitter at ground level and the signature signal generating device provided on the underground object provides for accurate detection of the subsurface object, despite the presence of a large background noise signal. The depth and, if desired, orientation of the underground object may also be determined using the signature signal generated by the signature signal generating device mounted to the underground object. Orientation information may be may be encoded on the signature signal or transmitted as an information signal separate from the signature signal. The probe signal may be microwave or acoustic. The signature signal produced by the signature signal generating device mounted to the underground object may be generated either passively or actively. Further, the signature signal bay may be produced in a manner which differs from the probe signal in one or more ways, including phase, frequency content, information content, or polarization. Also, the signature signal generating device may produce both location and orientation information, without the need for a separate orientation detecting device. Alternatively, orientation and location information may be produced by independent orientation detection and signature signal generating devices.
U.S. Pat. No. 5,942,899, issued Aug. 24, 1999, to Shrekenhamer et al, discloses a passive mine detection apparatus useful for searching out buried mines, exploits natural soil emissions at microwave frequencies and unique interference-induced spectral reflection signatures from planar surfaces of buried mines interacting with the soil emissions. The apparatus comprises a focused beam antenna, low noise amplifiers for respective polarizations, baseband converter, spectrum analyzer, A to D converter, signature recognition processor, display and/or alarm. Hand-held and vehicle-mounted implementations are disclosed.
U.S. Pat. No. 4,072,942, issued Feb. 7, 1978, to A. V. Alongi, discloses an apparatus for the detection of buried objects comprising a broadband, high resolution short pulse transmitter and a bistatic or monostatic noncontacting antenna for radiating the transmitted signal through the ground for reflection from a buried object, a sampling type receiver which reduces the bandwidth and center frequency of the received signal, and a locking circuit controlled by the first reflection from the ground or soil surface to thereby lock the range sweep to the soil surface and eliminate the effects of antenna height variations.
U.S. Pat. No. 4,937,580, issued Jun. 26, 1990, to R. H. Wills, discloses a ground probing radar for detecting radar reflections from underground objects. The radar is of the pulse compression type. A transmitter generates a biphase digitally modulated carrier signal. The digital modulations comprise successive pairs of complementary codes. Reflections of the transmitted signal from underground objects are demodulated and cross-correlated with the code words to produce a reflectivity sequence signal. The use of complementary codes results in minimal time sidelobes and improved range and resolution.
U.S. Pat. No. 4,062,010, issued Dec. 6, 1977, to Young et al., discloses an apparatus and method wherein an electrical impulse source transmits a radar-type signal through an antenna into the ground and is reflected by a target. The reflected signal or echo is detected by the antenna and an analog-to-digital converter converts it to a digital form which may be readily operated on, stored and recalled. A memory stores the information until recalled for comparison with a subsequent signal to give an indication of the location of metallic and non-metallic buried targets.
U.S. Pat. No. 5,363,050, issued Nov. 8, 1994, to Guo et al., discloses a microwave imaging system wherein a three dimensional profile of the dielectric permittivity of a target is obtained. A transmitter transmits microwaves toward a target, and the target scatters the microwaves. The scattered waves are received by an antenna and are converted into suitable data for application to a digital computer. The computer processes the data using either a scattering matrix algorithm or a Fourier transform algorithm. The computer then generates data representative of a three dimensional profile of dielectric permittivity which can be displayed on a suitable display device such as a CRT.
U.S. Pat. No. 3,775,765, issued Nov. 27, 1973, to Di Piazza et al., discloses a broadband, radar-type system for resolving the sizes and centroid locations of objects buried at a maximum depth in the order of 6 to 10 feet. The system uses a carrier frequency which is high enough so that an instantaneous bandwidth of about 25 percent provides resolution in the order of 1 foot. The system antenna includes impedance matching and focusing means. Polarization diversity of the transmitted beam may be accomplished to distinguish between elongated and generally round objects.
U.S. Pat. No. 4,746,867, issued May 24, 1988, to D. J. Gunton, discloses an antenna assembly for use in locating buried objects wherein particularly long thin objects such as pipes can be located, determining the position, and ascertaining the pipe direction by taking measurements from a single point, without mechanical movement of the antenna, and allowing a better suppression of spurious signals and reduction in false indications; which assembly has a plurality of antenna arms adapted and arranged to transmit and receive radiation into the ground and is characterized in that the arms have, on at least the surface nearest the ground, a cladding of a substantially lossless dielectric material.
U.S. Pat. No. 3,713,156, issued Jan. 23, 1973, to R. G. Pothier, discloses a detector apparatus in which the target area is illuminated by microwave energy in the millimeter range. A microwave lens element focuses the reflected millimeter waves to a focal plane. A microwave to thermal converter is disposed in the focal plane to convert the reflected microwave images to thermal images. A liquid crystal display or an IR area detector, such as a line scan unit is employed to convert the thermal images to a visible display of images in the target area.
U.S. Pat. No. 5,837,926, issued Nov. 17, 1998, to D. E. Franklin, discloses metal structures that are resonant to electromagnetic waves and combined with land mines to make them easier to detect using Ground Penetrating Radar. Knowledge of the resonant characteristics in the metal structures enhances detection and identification.
U.S. Pat. No. 5,680,048, issued Oct. 21, 1997, to W. T. Wollny, discloses a device that detects metallic and non-metallic objects on, flush with, or covered by the ground or other surfaces, or by interfering or obscuring structures or surfaces, using ground penetrating radar, a metal detector and a radiometer. It is specifically designed for detection of non-metallic mines. The coils of the metal detector are mounted in a multi-sensor module with the radar antenna in a co-boresighted and/or co-located arrangement, without degrading the performance of the metal detector or the ground penetrating radar. Preferably, the ground penetrating radar uses a feed and a collimation lens, (such as a Luneberg or Step Dielectric lens), as an antenna to reduce the change in the loss of signal strength due to changes in distance between the surface and the antenna (for short distances). The collimated beam has approximately constant power for distances closer than twice the diameter of the lens. By using the lens with a ground penetrating radar, the antenna can may be held somewhat farther from the ground, as well as eliminating Aclutter@ introduced as the antenna moves closer and farther from the ground. The sensor for the radiometer is co-located in the multi-sensor module. The sensors selected for the multi-sensor module employ different detection phenomena. Therefore, each sensor has its unique source for false alarms. The sensors=independent phenomenologies provide a synergism, which when processed, achieve an increase in probability of detection concurrent with a reduction in the false alarm rate for mines.
U.S. Pat. No. 5,051,748, issued Sep. 24, 1991, to Pichot et al., discloses a transmitting antenna which radiates a micro-wave field through an opening in the form of a rectangular wave-guide applied against the separation surface between a first medium, in which it is located, and a second medium, in which an object is buried. The microwave radiation reflected by the object is collected through the opening of a receiving antenna, also in the form of a rectangular wave-guide, applied against the radiating opening of the transmitting antenna. The collected radiation is measured at a series of points by means of pinpoint antennae located in the collecting opening. Thanks to the antennae arrangement, the collected radiation can be used as such, without having to subtract therefrom the result of a reference measurement. The invention can be used particularly to obtain, non-destructively, images of metal bars buried in reinforced concrete.
U.S. Pat. No. 5,420,589, issued May 30, 1995, to Wells et al., discloses a pulse radar system for determining the subsurface structure of a medium comprising an electronics unit for providing electronic signals and control comprising a utility controller, a sampler controller, and a timing controller such that the timing controller provides a pulsar trigger and the sampler controller provides a sampler trigger; a microwave unit comprising all the microwave components within the system including a pulsar for generating pulses as directed by the timing controller in the electronics unit, a transmitting antenna for receiving the pulses directly from and being in close proximity to the pulsar, a receiving antenna for accepting the pulses emitted from the transmitting antenna, and a receiver in close proximity to and for accepting the pulses from the receiving antenna, and a data unit for receiving signals from the electronics unit and for displaying the data for review and analysis.
U.S. Pat. No. 5,673,050, issued Sep. 30, 1997, to Moussally et al., discloses an ultra-wide band ground penetrating radar (GPR) system providing non-invasive detection and three-dimensional mapping of underground objects and voids. The performance of this radar provides improved underground object detection, location and identification over existing radars through the use of a novel interrupted, frequency modulated, continuous wave (FMCW) signal waveform. A synthetic aperture radar (SAR) technique known as spotlight mode focused (SMF) operation is used to collect data for the underground area of interest, by circumscribing this area with a radar beam provided on an airborne or ground based vehicle. Near-Brewster angle illumination of the ground is used to reduce losses.
U.S. Pat. No. 4,271,389, issued Jun. 2, 1981, to Jocobi et al, discloses that a physiologic facsimile image of a biological target without multipath contamination is obtained by first producing, for each one of a plurality of sample locations which are spaced so as to define a two-dimensional array, a time delay spectrum wherein the frequency of each spectral ordinate represents the instantaneous differential propagation delay between a first microwave signal which has been propagated through the target and a second microwave signal which initially corresponds to the first microwave signal, and which has been propagated through means having a predetermined propagation delay, and measuring the amplitude of the spectral ordinate corresponding to the direct ray path of propagation through the target, so as to obtain a set of data. The set of data is then digitized and converted from time domain to frequency domain. The transformed data is then processed is by sorting the data into column order; magnifying data derived from the sorting step so as to enhance and preserve the resolution of the image; mapping data derived from the magnifying step into further data using a predetermined mapping function so as to enhance the contrast between selected portions of the image; and obtaining a set of control signals which are used to actuate a display device to generate the facsimile image by filtering data derived from the mapping step using a band pass function which rejects spatial frequencies below a predetermined first frequency and/or rejects spatial frequencies above a predetermined second frequency so as to minimize, respectively, the effects of variations in the thickness of the target and/or spurious frequencies resulting from the magnifying step.
It is presently understood by the inventors that real world results for the above discussed prior art involving tests using actual soil and/or varied soil conditions have not produced repeatable and reliable success in locating underground plastic mines or non-metallic underground pipes. Therefore, those skilled in the art have long sought and will appreciate the present invention that addresses these and other problems.
The present invention provides apparatus and methods for detecting, locating, and identifying concealed objects and to measure small differences in interfaces between liquid/gases and different geological formations. Data responsive to these physical phenomena is acquired using microwave equipment such as a transmitter/receiver and a wide bandwidth antenna. Novel processing techniques provide information about concealed objects that may be disposed in environments, such as soil, that have effectively shielded concealed objects using microwave techniques. One intended use of the invention is to locate and identify land mines, particularly small antipersonnel plastic land mines. Other uses include, but are not limited to, detecting underground plastic pipes, detecting changes in liquid/foam interfaces within storage containers, detecting anomalies in microwave penetrable materials, detecting interfaces between geological materials, e.g., quartz/rocks, phosphates/soil, detecting voids, and the like.
In one embodiment of the invention, a method of detecting anomalies in a microwave penetrable material is provided that comprises steps such as transmitting a microwave signal into the microwave penetrable material that is stepped over a plurality of frequencies at each of a plurality of different positions. A plurality of reflections are received for each of the plurality of frequencies transmitted for each of the plurality of positions. Each of the plurality of reflections have a magnitude and a phase and a time delay that is preferably measured and stored. The magnitude and phase and time delay for each of the plurality of target reflections are utilized to produce a complex target data vector at each of the plurality of positions. For purposes herein, a complex data vector is considered to be a matrix or group of values where one or more values includes complex numbers. The magnitude and phase and time delay for the reference reflections at the one or more of the plurality of positions may be used to produce a complex reference data vector. The complex reference data vectors may compared with respect to the complex target data vector to produce a complex channel vector with respect to the plurality of frequencies for each of the plurality of positions. A complex filter matrix of values corresponding to the plurality of frequencies and the time delays may be determined. The complex filter matrix may be constant for each of the plurality of positions. The complex filter matrix may preferably be a least squares operator useful for predicting reflection amplitude. The complex filter matrix may be multiplied times the complex channel vector and divided by the number of the plurality of frequencies to obtain a response signal with respect to time delay for each of the plurality of positions. The anomalies in the microwave material may then be detected from changes in the response signal with respect for each of the plurality of target points.
Accelerometers may be used to determine the relative location of the plurality of positions in the microwave penetrable material.
In another embodiment, the method comprises utilizing the magnitude and phase and time delay for each of the plurality of reflections to produce a complex target data vector for each of the plurality of positions. The magnitude and phase and time delay for one or more of the plurality of reflections at the one or more of the plurality of positions may be used to produce a complex reference data vector. The complex reference data vector may be compared with respect to the complex target data vector to produce a complex channel vector for each of the plurality of positions. A complex conjugate of the complex channel vector may added to the complex channel vector for each of the plurality of reflections to form a conjugate symmetric complex channel vector. Then an inverse Fourier transform is taken of the conjugate symmetric complex channel vector to thereby produce an impulse function such that impulses are produced located at respective reflection time delays that indicate the anomalies in the microwave material. In a preferred embodiment of this method, the complex reference data is taken on the fly and may be updated so as to remain within about six inches from the transmitter.
In another embodiment, a purely theoretical reference signal may be predetermined for use. The theoretical reference signal is compared to the complex target data vector for each of the plurality of frequencies at each of the plurality of positions to produce an error signal that is used for detecting the anomalies from changes in values of the error signal at each of the plurality of positions. In a preferred embodiment, the theoretical reference signal is determined to be equal to a first quantity of a frequency dependent soil frequency minus an impedance of air divided by a second quantity of the frequency dependent soil frequency plus the impedance of air.
In another embodiment, a complex reference signal is compared with the complex antenna impedance for each of the plurality of reflections at each of the plurality of positions to produce a complex difference signal for each of the plurality of reflections at each of the plurality of positions. The complex difference signal is amplified for each of the plurality of reflections at each of the plurality of positions to produce an amplified complex difference signal. A Fourier transform is taken of the complex difference signal to thereby produce a response signal. In one form of this embodiment, the complex reference signal further comprises determining an average complex antenna impedance for at least two of the plurality of reflections for at least one of the plurality of positions. In another embodiment, a theoretical value for the complex reference signal may be used. Depending on a preferred feature of the embodiment, the complex reference signal may vary over the plurality of positions or remain constant.
An object of the present invention is to detect anomalies such as plastic mines or pipes within a soil environment.
This and other objects, features, and advantages of the present invention will become apparent from the drawings, the descriptions given herein, and the appended claims. It will be understood that any listed objects of the invention are intended only as an aid in understanding aspects of the invention and are not intended to limit the invention in any way.